DNA methylation profile of the mouse skeletal alpha-actin promoter during development and differentiation.

Genomic levels of DNA methylation undergo widespread alterations in early embryonic development. However, changes in embryonic methylation have proven difficult to study at the level of single-copy genes due to the small amount of tissue available for assay. This study provides the first detailed analysis of the methylation state of a tissue-specific gene through early development and differentiation. Using bisulfite sequencing, we mapped the methylation profile of the tissue-specific mouse skeletal alpha-actin promoter at all stages of development, from gametes to postimplantation embryos. We show that the alpha-actin promoter, which is fully methylated in the sperm and essentially unmethylated in the oocyte, undergoes a general demethylation from morula to blastocyst stages, although the blastula is not completely demethylated. Remethylation of the alpha-actin promoter occurs after implantation in a stochastic pattern, with some molecules being extensively methylated and others sparsely methylated. Moreover, we demonstrate that tissue-specific expression of the skeletal alpha-actin gene in the adult mouse does not correlate with the methylation state of the promoter, as we find a similar low level of methylation in both expressing and one of the two nonexpressing tissues tested. However, a subset of CpG sites within the skeletal alpha-actin promoter are preferentially methylated in liver, a nonexpressing tissue.